1. Field of the Invention
The invention relates to an air mass meter for the intake port of an internal combustion engine, with a temperature sensor which records the temperature of the air in the intake port, and with a heating element which has a strip-shaped substrate and a heating resistor applied to one side of the substrate.
An earlier, commonly owned international application (WO 95/21370) describes an air mass meter which has a temperature sensor resistor and a sensor heating resistor. The sensor heating resistor is heated to a predetermined excess temperature, as compared with the temperature measured at the temperature sensor resistor. The electric current, which flows through the sensor heating resistor, and the voltage, which drops across a temperature-independent series resistor, are, when squared, directly proportional to the heating capacity which is delivered in the sensor heating resistor. The air mass flow can thus be determined from these variables.
Deposits of dirt on the onflow surface of the sensor heating resistor which face into the flow may lead to variations in heat transfer on the sensor heating resistor. The increasing deposits of dirt result in an increasingly greater measuring error.
A prior art air mass meter described in German published, non-prosecuted patent application DE 38 38 466 A1 has a temperature sensor and a heating element with a striplike substrate and a heating resistor applied to the substrate. A body member is arranged on one end face of the substrate, facing into the flow of the air. That body member is disposed and configured such that, when the air mass meter is new, a flow profile is already established around the substrate which resembles the flow profile that is established, without the body, as a result of deposits of dirt after a certain amount of operating time. Deposits of dirt on the body member may, however, lead to a variation in heat transfer on the heating resistor. Increasing deposits of dirt thereby result in an increasingly greater measuring error.
For the purpose of eliminating deposits of dirt, a prior art air mass meter described in German patent DE 42 28 524 C1 has a U-shaped yoke which is mounted movably in a frame. A first free end region brushes against the onflow surface of the sensor heating resistor. A second free end region of the yoke is driven by the air flow in the intake port. The position of the first free end region consequently varies with the velocity of the air flow. The first end region of the yoke thus eliminates deposits of dirt on the onflow surface mechanically. The disadvantage of this air mass meter, however, is that is requires complicated mechanical parts in order to eliminate the deposits of dirt.